I have the same model xr8 and have been reseaching the exhaust side of things for a while now. Just about to start the mod process actually.
Firstly if they guy at the shop has already told you openly that he doesn't know what he is doing. Dont go back there. Go to some one that knows and has done them before.
What I have found so far:
Best Extractors are apparantly
PACEMAKER COMPETITION (extractors)
Best Air Filter
BMC (yep more Kw's than a k&n)
Best Exhaust system
This is a split decision between
1) dual 2 1/2" pipe work
2)single 3" pipe work.
I will be using a dual system 2 1/2" with gutted cats(naughty naughty) & 2 Lukey Race magnum mufflers(straight through)right up the back/Pacemaker Competition Extractors/chiptorque custom chip/Bmc Air Filter and possibly modify the Cold air intake.
And hopefully all before the end of December. CANT WAIT